1. Field of the Invention
The present invention relates to a wireless communication system and a wireless communication apparatus by which a plurality of wireless communication apparatuses make wireless communication with one another by sharing a plurality of different frequency channels.
2. Description of the Related Art
Media access control (MAC) is control that determines how a plurality of communication apparatuses for communicating by sharing the same medium should utilize the medium and transmit communication data. As a result of performing MAC, even if two or more communication apparatuses simultaneously transmit communication data by utilizing the same medium, occurrences of events (so-called collisions) such that a communication apparatus on a receiving side can not make the communication data separable are decreased. An event such that the medium is not used by any communication apparatus regardless of existence of a communication apparatus having a request for transmission is also decreased as a consequence of performing MAC.
In wireless communication, since it is hard for the communication apparatus to monitor transmission data while transmitting the transmission data, MAC without assuming detection of the collision is required. IEEE 802.11 being a representative technical standard for a wireless local area network (LAN) adopts carrier sense multiple access with collision avoidance (CSMA/CA).
CSMA/CA in IEEE 802.11, sets a period (referred to as duration) until a series of sequences composed of one or more frame exchange following a MAC frame to a header of the MAC frame. During the duration, the communication apparatus having little relation to and having no transmission right of the sequences stands ready for transmission by determining a virtual occupancy state of the medium. Thereby, the occurrence of the collision is avoided. In contrast, the communication apparatus having the transmission right in the sequences comes to know that the medium is not used other than the period in which the medium is actually occupied.
IEEE 802.11 defines that a state of the medium is determined by a combination of virtual carrier sense in a MAC layer such as the former and physical carrier sense in a physical (PHY) layer such as the latter and the MAC is executed on the basis of the determination.
Jpn. Pat. Appln. KOKAI Publication No. 2003-87856 discloses a method for achieving a wireless base station capable of being shared in a plurality of wireless LAN systems in a system in which a plurality of wireless LAN systems differing in PHY layer are coexisting. Specifically, the wireless base station alternately generates a first notification signal in a first PHY layer and a second notification signal in a second PHY layer to transmit them to a wireless terminal and switches over the first and the second PHY layers in synchronous with the first and the second notification signals. The base station makes it possible for the wireless terminal corresponding to the first PHY layer to access only within a specified time from the transmission time of the first notification signal and makes it possible for the wireless terminal corresponding to the second PHY layer to access only within a specified time from the transmission time of the second notification signal.
IEEE 802.11 adopting CSMA/CA has been speeding up communication speed by mainly changing a protocol of a PHY layer up to this day. For the 2.4-GHz band, IEEE 802.11 (established in 1997, communication speed=2 Mbps) has changed to IEEE 802.11b (established in 1999, communication speed=11 Mbps) and further to IEEE 802.11g (established in 2003, communication speed=54 Mbps). For the 5-GHz band, only IEEE 802.11a (established in 1999, communication speed=54 Mbps) currently exists as a standard specification.
As for one approach to speed up the communication speed, a method of increasing the bandwidth of a channel is usable. In a method of expanding the bandwidth of the channel, a channel having a certain bandwidth and another channel having a bandwidth greater than that of the certain channel coexist in a certain frequency band. Conventional CSMA/CA, however, is designed so as to perform access control for a single channel, so that it cannot perform MAC for a plurality of channels having different bandwidths.
The easiest MAC system for a plurality of channels having different bandwidths coexisting in the same frequency band includes a system in which a central control station manages every MAC. For example, in the IEEE 802 wireless LAN system, an access point (AP) can manage every MAC. In recent years, an ad hoc network for directly transmitting and receiving data to be transmitted from one mobile station to another mobile station without passing though the central control station is paid attention in an advantage to enable preventing transmission efficiency from being degraded due to the passing of the data through the central control station.
Such a wireless communication system in which the central control station does not exist, however, cannot use the resolving means described above.